Every year, from 44,000 to 98,000 people in the United States die as a result of medical errors. One common medical error involves medication error (e.g., prescribing or administering the wrong medication, incorrect dosing, drug interactions, failure to order therapeutic monitoring tests). Data on medication errors are collected predominantly in hospitals, where it is easier to monitor such errors and to identify patients experiencing adverse drug events. However, in the United States, most medications are prescribed by primary care physicians in the outpatient setting. While computerized physician order entry systems (CPOE) have been shown to decrease medication errors in the inpatient setting, their effect at the point of care in the outpatient setting has not been well studied. The specific aim of this two-year study is to determine the frequency, type, severity, and preventability of medication errors in primary care settings that use an electronic medical record (EMR), with advanced decision support at the point of care, compared with primary care settings where a paper medical record (PMR), without advanced decision support, is used. The study will be conducted in four clinical settings, each representing one constituent organization of the Southern Primary Care Urban Research Network (SPUR-Net(SM)), which is located in Houston, Texas, and administered by Baylor College of Medicine. Two sites use an EMR, Logician(R), and two use PMR. The study will proceed in three phases. In phase I, a random sample of 500 adult patients (125 from each site) who receive a new prescription at the index study visit will be recruited. Consent will be obtained for a medical record review (phase II), where diagnostic codes (ICD-9), medications, and toxic/therapeutic monitoring test results will be abstracted. Phase III will involve a telephone interview occurring within two weeks of the index visit to assess prescription fill and significant symptoms experienced. Medication errors will be identified and classified using a modified version of the Brigham and Women's Medication Safety Protocol and the National Coordinating Council for Medication Error Reporting and Prevention Classification of Medication Errors. Inter-rater reliability will be assessed in a random 10-percent sample of patients. Several of the SPUR-Net partnering organizations have adopted an EMR, while other partners have plans for implementing an EMR in the future. We therefore have the opportunity to use this natural experiment to test the impact of an EMR, compared with a PMR, in reducing medication errors in primary care settings. The proposed research is innovative, as it will provide an understanding of medication errors in a diverse primary care population and assess the effect of computerized physician order entry system with computerized decision support in decreasing medication errors. Moreover, these outcomes will fundamentally advance our knowledge and the promotion of medication safety.